Conventional motor vehicles using internal combustion engines are operated using a four-stroke cycle, also termed Otto cycle. One problem with internal combustion engines is the low thermal efficiency of the Otto cycle. Thermal efficiency is defined as the useful work output of an engine divided by the fuel consumed at its lower heating value. The peak thermal efficiency for a spark-ignited (SI) engine may be 32% in an automotive application, but the thermal efficiency of the same engine at its normal operating point may be only 15 to 20% or lower. In the 20% thermal efficiency case, 80% of the heat energy in the fuel is discarded as waste heat and is not converted to useful work. In a conventional engine, this heat is lost through the exhaust and through the cooling system, whether the engine is air cooled or liquid cooled. Friction inside the engine also accounts for 10 to 25% of the gross work output from the engine. This friction ends up as heat exiting the engine, so this friction is already included in the peak efficiency figures given. There are ways of increasing the thermal efficiency of an engine. In order to improve the thermal efficiency of internal combustion engines it has been suggested that the use of a six-stroke cycle would be advantageous to use some of the waste heat normally expelled in the exhaust.
For instance, US2012/0060493 discloses a six-stroke engine where water is injected into the cylinder after the exhaust stroke and is instantly turned to steam, which expands and forces the piston down for an additional power stroke. Thus, waste heat that requires an air or water cooling system to discharge in most engines is captured and put to use driving the piston. Hence the need for a cooling system can at least be reduced, if not eliminated. The weight associated with a cooling system could be eliminated, but that would be balanced by a need for a water tank in addition to the normal fuel tank.
Although known solutions can extract additional power from the expansion of steam and improve the fuel consumption of a conventional four-stroke engine, there is a need for further improving the thermal efficiency of internal combustion engines.
An object of the disclosure is to provide an internal combustion engine and a method for operating such an internal combustion engine in a six-stroke mode in order to improve the thermal efficiency of the engine.